1. Technical Field
The present disclosure relates generally to radio frequency (RF) signal circuitry, and more particularly, to coupled resonator on-die filters for WiFi applications.
2. Related Art
Wireless communications systems are utilized in a variety contexts involving information transfer over long and short distances alike, and a wide range of modalities for addressing the particular needs of each being known in the art. As a general matter, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent information/data, and the encoding, modulation, transmission, reception, de-modulation, and decoding of the signal conform to a set of standards for coordination of the same.
In the local area data networking context, WLAN or Wireless LAN, also commonly referred to as WiFi as well as 802.11 (referring to the governing IEEE standard), is the most widely deployed. The WiFi standard specifies a time domain duplex system where a bi-directional link is emulated on a time-divided communications channel. Several computer systems or network nodes within a local area can connect to an access point, which in turn may provide a link to other networks and the greater global Internet network. Computing devices of all form factors, from mobile phones, tablets, and personal computers now have WiFi connectivity, and WiFi networks may be found everywhere.
As is fundamental to any wireless communications systems, a WiFi network interface device includes a transceiver, that is, a combined transmitter and receiver circuitry. The transceiver, with its digital baseband system, encodes the digital data to an analog baseband signal, and modulates the baseband signal with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transceiver converts the electrical signal to electromagnetic waves, and vice versa. In most cases, the transceiver circuitry itself does not generate sufficient power or have sufficient sensitivity necessary for communications. Thus, additional circuits are referred to as a front end is utilized between the transceiver and the antenna. The front end includes a power amplifier for boosting transmission power, and/or a low noise amplifier to increase reception sensitivity.
Allocations and other usage restrictions of the RF spectrum are established on a region-by-region basis by governmental agencies having jurisdiction in those particular regions. In the United States, the Federal Communications Commission is the responsible agency. One widely used frequency allocation is the Industrial-Scientific-Medical (ISM) band, and WiFi systems utilize the 2.4 GHz frequency in the ISM band (referred to herein as the 2 GHz band). More recent iterations of the IEEE WLAN standard also specify the use of the 5 GHz operating frequency in the ISM band, for which usage has been licensed.
A typical multimode WLAN transceiver thus has a separate 5 GHz module and a 2 GHz module, with separate inputs, outputs, and enable lines for activating the transmit and receive amplification functions. One variation involves the use of two separate antennas for each operating frequency. That is, there may be 5 GHz antenna to which a power amplifier circuit and a low noise amplifier circuit specific to the 5 GHz transmission and reception are selectively connected over a single pole, double throw switch. There may also be a 2 GHz antenna to which a power amplifier circuit and a low noise amplifier circuit specific to the 2 GHz transmission and reception are selectively connected over another single pole, double throw switch. There may be various co-existence filters at the input of the respective power amplifier circuits as well as at the antennas. Another variation relies upon a single antenna for both receive and transmit functions in both operating frequencies. In such a configuration, there may be a duplexer connected to the single antenna that is connected to the respective transmit/receive switches for each operating frequency.
During concurrent operation, the 5 GHz transceiver and related circuitry, and the 2 GHz transceiver and related circuitry, can generate high levels of unwanted emissions for counterpart receivers. In the aforementioned WLAN system, whether dual antenna or single antenna, signals generated at the 5 GHz power amplifier can interfere with 2 GHz low noise amplifier. Conversely, the signals generated at the 2 GHz power amplifier can interfere with the 5 GHz low noise amplifier. Furthermore, there may be interference as between the two switches, the output ports of the front end circuit, and the antennas. WiFi connectivity is oftentimes not the only RF signal source in most multi-function devices, and there are other wireless communications modalities such as cellular telephone transceivers and GPS (Global Positioning System) receivers in close proximity. In such case, interference from and interference to these other modalities may also occur.
The 5 GHz WLAN transceivers tend to exhibit a high level of local oscillator spurs at the 3.2 GHz to 3.9 GHz range, as well as the 6.8 GHz to 7.8 GHz range. Similarly, the 2 GHz WLAN transceivers have a fairly high level of local oscillator spurs at the 1.6 GHz to 1.7 GHz range, as well as the 3.2 GHz to 3.5 GHz range. It is thus desirable for the front end circuit to reject these frequencies, partly in order to meet FCC spectrum emission requirements, and partly to minimize receiver desensitization in other communication modalities. Indeed, if the transmit chain has sufficient gain, these spurious emissions may be amplified and transmitted such that government-mandated co-existence parameters are exceeded and would limit or prevent the operation of other wireless systems. Furthermore, there are restricted bands in which transceiver circuitry cannot exhibit more than −41.5 dB/MHz of spurious emissions, and the aforementioned spurs of may be within such restricted bands. Even in unrestricted frequency bands, different locales may limit the level of allowed emissions.
Various filters implemented in the front end module for rejecting these spurious emissions in WLAN applications are known in the art. One example is a differential filter based on bond wires, though parasitic coupling between inductors in the circuit are avoided. However, such filters typically have high insertion loss on the order of 10 dB or greater, and the matching characteristics are problematic. One known implementation has an input return loss (S11) of −3 dB in-band. Accordingly, there is a need in the art for filters with improved rejection characteristics for unwanted emissions generated from dual band WiFi systems in close proximity to the pass band, with minimal loss, and better matching characteristics.